The invention relates in general to an arc tube assembly for a high intensity metal halide lamp and, particularly, to such an assembly having improved structural and mechanical integrity. More particularly, the invention relates to an arc tube assembly for a metal halide lamp which consists of a glass arc discharge envelope and a pair of side arms attached thereto, each arm containing a specially constructed electrode assembly.
High pressure mercury vapor discharge lamps are well known in the prior art. See for example, U.S. Pat. No. 3,654,506 issued to Kuehl, et al. Other prior art is shown in U.S. Pat. Nos. 4,559,472, 4,376,906, 3,868,528, 3,753,026, 4,219,757, 4,423,353, 4,254,356, 3,205,395, and 3,351,803. These patents all relate to gas discharge lamps and various components thereof. High pressure discharge lamps typically include elongated electrodes extending into a gas discharge space formed by a glass enclosure. The discharge space includes a rare gas and the electrodes form respectively a cathode and an anode, as is well understood by those skilled in the art. The electrodes lead outwardly from the glass enclosure and must be sealed hermetically thereto.
In typical high intensity metal halide lamps, a vitreous quartz arc tube discharge envelope is filled with argon gas, mercury, plus other metal salts. Protruding into the arc tube discharge envelope are two tungsten electrodes, each of which form a part of an electrode assembly. Each electrode is connected to molybdenum foil which is, in turn, connected to a follower rod, which serves as an electrical termination. The tungsten electrode assemblies including the molybdenum foil are generally, but not always, concentric with the axis of the arc tube discharge and located at opposite extremes of the arc tube discharge envelope. Each tungsten electrode assembly, including the molybdenum foil is encased in a vitreous quartz tube.
While within the sealed arc tube discharge envelope, high electrical currents are conducted to the discharge envelope through the follower rod and molybdenum foil conductors. In addition to providing the means for conducting electrical excitation to the arc tube discharge envelope, the quartz arms containing the molybdenum foil attached to the tungsten electrodes also provide the mechanical support for the arc tube discharge envelope.
It has been found that lamps of this high intensity type often fail if the seal between the vitreous quartz of each arm and the molybdenum foil conductor contained therein is not absolutely perfect. That is, in the manufacture of such lamps, intimate contact between the molybdenum foil and the vitreous quartz tubular arm must be assured. If a gap or space exists between the molybdenum foil and the vitreous quartz, a high probability exists that a crack will be formed within the vitreous quartz as a consequence of the high currents flowing within the molybdenum foil, thus causing ultimate failure of the lamps.
Manufacturing processes and structures have been suggested in the prior art to achieve a molybdenum ribbon/vitreous quartz high integrity seal. One commonly used sealing method is the pinch or press seal. In the pinch or press operation the electrode ribbon assembly is supported within a thin wall quartz tube. A two-part ring burner heats the quartz tubing to soften it, and a pair of opposing pinch jaws strike the soft quartz tubing and seal it firmly to the ribbon, leaving one end of the ribbon exposed to the air outside the seal. A pinch seal has little mechanical strength and cannot be used to support the relatively heavy discharge envelope, particularly when lamps of large size are being made.
To make a molybdenum ribbon seal air compatible, i.e., to prevent oxidation of the molybdenum ribbon, it is necessary to make it long enough to ensure that the end exposed to air is operating at a temperature low enough to preclude oxidation of the molybdenum quartz interface, or to provide a second seal which prevents the oxidizing atmosphere from reaching the seal.
These problems are addressed, e.g., in Buchwald U.S. Pat. No. 3,205,395 dated Sept. 7, 1965, which teaches the construction of a high intensity lamp by using a quartz stem pressed tube incorporating a second seal to prevent oxidation of the molybdenum ribbon seal. Moreover, Buchwald teaches the insertion of the quartz stem pressed into a vitreous quartz tube arm of slightly larger inside diameter than the outside dimension of the stem pressed tube. This built-up assembly provides the mechanical structure which constitutes the arm attached to the discharge envelope. A gap thus results between the quartz of the stem pressed tube and the exterior quartz arm. This discontinuity in quartz is not beneficial to the performance of the lamp, reducing the structural integrity of the arm while not providing the heat dissipation needed for high current operation.
In an attempt to form an improved seal between the molybdenum foil and the vitreous quartz, vacuum shrinking of the quartz arm onto the molybdenum foil electrode assembly has been attempted. Typically, an electrode assembly consisting of the tungsten electrode attached to the molybdenum foil in turn attached to the electrical terminator is inserted within a relatively thick-walled vitreous quartz tube. Using techniques known in the art, the vitreous quartz tube is vacuum shrunk about the molybdenum foil electrode assembly upon application of suitable heat. This improvement over the stem pressed structure can be made long enough to cause the exposed end to operate below the oxidation temperature of the molybdenum ribbon, thereby eliminating the necessity of a second seal. However, it has several practical drawbacks.
To achieve the strength necessary to support the discharge envelope the thickness of the vitreous quartz which must be shrunk onto the molybdenum foil is on the order of three millimeters. It is extremely difficult to heat such a thick-walled cylinder uniformly due to slight variations in the wall of the quartz tube. Because of the poor heat conducting properties of quartz, a large temperature gradient appears between the outside surface of the tube where heat is applied and the inside surface where the seal is to be made. A temperature substantially in excess of the softening point is needed at the inside wall to ensure that the quartz flows uniformly onto the ribbon and into all voids. To achieve this flow on the inside surface requires that the outside surface be molten and extremely free flowing. The heating and vacuum shrinking of such thick-walled vitreous quartz about the molybdenum foil is extremely delicate, and it has been performed in practice only by skilled artisans through manual operation. Due to the delicacy of the operation, it has not been possible to automate it with any degree of success.
A further problem in sealing heavy side arms is the residual strain left in the quartz subsequent to the sealing. Although annealing removes a good portion of the strain, subsequent thermal cycling in the operation of the lamp can introduce new strains in the side arms. These strains can build to the point of causing a crack in the side arm which invariably destroys the integrity of the seal and causes the lamp to fail. Even utilizing handmade techniques which are extremely expensive, high intensity lamps of the type manufactured by heat and vacuum shrinking thick-walled quartz about the molybdenum foil often fail due to molybdenum foil vitreous quartz seal defects.
Another attempt to solve the problem of unsatisfactory foil to glass seals is the use of an electrode assembly having two molybdenum foils separated by a thin quartz plate sealed in a lamp arm. This configuration increases the current carrying capacity of the electrode assembly in the arm; however, it is nonsymmetrical, thereby causing strain in the arm/foil assembly when the lamp thermally cycles in the operation of the lamp.
A further attempt involves the use of an electrode assembly having up to three molybdenum foils sealed in a lamp arm. Such assemblies use a quartz tube for the inner surface and a larger quartz tube for the lamp arm, and vacuum shrink seal the three foils between the two tubes. This system, however, causes cracks to occur in the foil at the transition at the electrode to foil area, so that mechanical strength and current carrying capacity are limited.
Attempts have also been made to use a prebeaded electrode assembly. Such prebead assemblies entail an extra labor and material step that renders the assembly uneconomical.
The present invention is an improved arc tube assembly for a high intensity metal halide lamp assembly consisting of a glass arc discharge envelope and a pair of side arms attached thereto, each of which side arms contain an electrode assembly of a unique and novel design sealed hermetically therein. The novel electrode assembly allows an arc tube assembly to be made having substantially greater structural and mechanical integrity as compared to the assemblies of the prior art.
It is an object of this invention to provide an improved arc tube assembly for use in connection with a high intensity metal halide lamp.
It is a further object of this invention to provide an improved arc tube assembly which includes 4 molybdenum foils hermetically sealed in the arm of a glass arc discharge envelope.
It is a further object of this invention to provide a method of making the improved arc tube assembly of this invention.
It is a further object of this invention to provide a method of making a foil to glass seal for use in an arc tube assembly.
It is a further object of this invention to provide an improved arc tube assembly which has substantially greater structural and mechanical integrity as compared to the assemblies of the prior art.
It is a further object of this invention to provide a means for connecting molybdenum foil to electrodes and electrical leads.
It is a further object of this invention to provide a means for placing molybdenum foil used in electrode assemblies under tension while undergoing shrink sealing.
It is a further object of this invention to provide a new and improved electrode assembly for use in connection with an arc tube assembly forming a part of a high intensity metal halide lamp.
The improved arc tube assembly of this invention comprises a vitreous quartz arc tube discharge envelope assembly having a pair of tubular side arms forming a part thereof and a pair of "electrode-foil-slug-outer lead" assemblies (hereinafter referred to as "electrode assembly") mounted therein. The discharge assembly comprises a gas discharge envelope to which are attached a pair of side arm tubes with capillaries. The gas discharge envelope can be tubular or globular in shape while the arms are generally tubular in shape and coaxial with one another with the side arms located on opposite sides of the discharge envelope and integrally fused to the discharge envelope. Each side arm is adapted to receive one electrode assembly of this invention into it.
The term "capillaries" is known to those skilled in the art and refers to a reduced inner dimension section of the side arm tubes at their juncture with the discharge envelope through which electrodes protrude, the outer dimensions of each electrode corresponding to the inner dimension of each capillary such that a mating relationship is achieved when the electrode projects therethrough.
Each electrode assembly is constructed from an electrode, four foil ribbon assemblies, each of which is attached to the electrode, a quartz slug forming a core for the assembly which the ribbon assemblies overlie, and four outer leads attached to the foil ribbon assemblies, all of which is intimately sealed into a side arm in the finished arc tube assembly of this invention.
The foil ribbon assembly is constructed from individual molybdenum foil ribbons specially cut and folded at one end, being connected to the electrode by a platinum weld, while on the other end is attached an outer lead (electrical terminator) by means of a platinized molybdenum interface. Each of the outer leads (electrical terminators) is constructed from molybdenum wire cut to length and formed to the desired shape. The quartz slug is constructed from a quartz rod using special fixtures and standard quartz shaping techniques to obtain the desired shape. It is then fire glazed.
In the manufacture of the arc tube assembly of this invention the arc discharge envelope with side arms attached is placed into an automated sealing machine. One electrode assembly is inserted into a side arm with the electrode projecting through the capillary section of the arm into the discharge envelope. At this point, the first arm is automatically vacuum shrink sealed to the electrode assembly. Next the second electrode assembly is inserted into the second arm in the same manner as the first. At this point the second arm is automatically vacuum shrink sealed to the second electrode assembly. Since the quartz can move very little in this configuration, there is an excellent quartz to multiple foil seal that can now be automated which is heretofore unknown in prior art.
Other objects and advantages of the present invention will be apparent to those skilled in the art from the claims and, with reference to the drawings, the following description of a preferred embodiment.